1. Field of the Invention
The present invention relates to a printing apparatus including an ink supply device such as an ink duct device. Particularly the present invention relates to a technique for performing quality control on printed paper sheet in the printing apparatus.
2. Description of the Background Art
A typical offset printing apparatus includes a plurality of ink duct (or ink fountain) devices having ink keys, and independently controls the ink duct devices to adjust the amount of ink supplied to respective segments extending across a predetermined feed direction of a printed paper sheet. This controls the amount of ink in accordance with the area of an image on a printing plate. With a conventional printing apparatus as described above, an operator suitably takes a sample from printed paper sheets, measures the printed density and printed color of each of the regions of the sample sheet on a separately provided table for use in measurement, and effects feedback control of the amount of ink supplied to each of the regions, based on the measurements.
However, the above-mentioned process, which is based on the premise that the operator performs sampling inspection, reduces inspection frequency, and might potentially result in omission of inspection of some samples during printing because of the absence of the operator. In such a situation, if an unexpected or sudden problem arises from, for example, an excessive or insufficient supply of ink, the detection of the problem is delayed.
To solve the above-mentioned problem, a printing apparatus known in the art comprises a measuring device for measuring printed materials. Provision of the measuring device in the printing apparatus allows approximately real-time ink supply control, to drastically reduce the burdens on the operator.
Recently, there has been a growing understanding of the quality control of printed materials, and the demands of clients for product quality has been increasing. The printing apparatus comprising the above-mentioned measuring device is capable of effecting ink supply control based on measurement data, but does not further utilize the measurement data.
The present invention is intended for a printing apparatus including an ink supply device such as an ink duct device.
According to the present invention, the printing apparatus for applying ink to a printing medium while feeding the printing medium in a predetermined feed direction, thereby to provide a printed sheet, comprises: an ink supply mechanism for individually supplying a designated amount of ink through a plurality of ink transfer mechanisms to a plurality of regions defined on the printing medium to obtain a printed sheet, each of the regions extending in the feed direction; an image reader provided in a feed path of printed sheets for capturing an image on a sample sheet selected among the printed sheets to obtain image data; an computation device for processing the image data to compute measurement data about the sample sheet for each of the regions; a controller for controlling the image reader to read image data and controlling the computation device to compute measurement data from the obtained image data while intermittently selecting the sample sheet among the printed sheets; and a storage element for storing the measurement data for each of the regions in association with printing history information.
Storing the measurement data including a printed density in association with the number of printed sheets or time allows history management to be performed on the measurement data accurately and easily.
Preferably, the printing apparatus further comprises an output device visually outputting the measurement data in a historical form.
Preferably, in the printing apparatus, the computation device is operable to compute representative data including at least one of a maximum value, a minimum value and an average value of the measurement data, whereby the output device visually outputs a result of computation performed by the computation device.
It is therefore an object of the present invention to provide a printing apparatus capable of using (displaying or printing) measurement data as history data for quality control to satisfy a requirement.